The long-term goal of my proposal is to understand how RNA-protein complexes regulate translation to influence cell fate. The C. elegans adult germ line provides a simple model to study questions involving the molecular controls influencing a germ stem cell's (GSC's) decision to proliferate or differentiate into gametes. The GSC fate decision is coordinated by FBF-1 and FBF-2 (collectively referred to as FBF), redundant members of the highly conserved PUF (Pumilio and FBF) protein family. FBF binds to specific messenger RNAs (mRNAs) necessary for differentiation, and is shown in vitro to associate with deadenylating (CCF-1/Not deadenylation complex) and polyadenylating (GLD-3/GLD-2 cytoplasmic poly(A) polymerase) proteins. In a working model, FBF interacts with specific poly(A) tail modifying proteins in different regions of the germ line to regulate expression of target mRNAs involved in GSC fate decisions. The following proposal combines established techniques in genetics, cell biology, and biochemistry, with newer developments in fluorescent microscopy, to further our understanding of the assembly and spatial regulation of FBF complexes. I will test our model with three specific aims: 1) I will determine the specific protein combinations that can assemble with FBF in a minimal in vitro protein interaction network. 2) Using advanced fluorescent imaging techniques, I will visualize where in the germ line and what subcellular location FBF interacts with its binding partners. 3) I will test in vivo whether FBF adenylation-modifying protein complexes affect the poly(A) tail length of FBF- associated transcripts. Together, my proposal will increase our general understanding of how RNA regulatory protein complexes control tissue development in a multi-cellular organism.